The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and ... [more ▼]

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and diffraction effects. In particular, small-scale ionospheric irregularities generated by different physical processes may cause scattering effects on GNSS signals, producing rapid fluctuations of the signal phase and amplitude as a result. Such scintillations of GNSS signals are responsible for critical consequences regarding applications, such as precise positioning, due to many resulting effects: cycle slips, signal power fading, receiver loss of lock and poor resulting satellite geometry. Ionospheric Scintillation Monitoring Receivers collect high-rate GNSS data. Specific scintillation parameters, such as the well-known S4 and Phi60 indices, are built on high-rate measurements performed on GNSS signals and provide additional information to characterize the intensity of such an event occurring at a specific geographic location at a given time. Spatial Statistics belong to the field of Spatial Analysis, Geography and GIS (Geographic Information System). This discipline allows to perform analyses of data which are localised in space. Ionospheric Scintillation observations achieved by ISMR stations can be characterized by a set of attributes (S4, Phi60, Rate of TEC, etc.) including also the geographic location of their respective Ionospheric Pierce Point (IPP). By combining the simultaneous Multi-GNSS ISMR measurements from a network of ISMR stations, we can obtain a spatially denser data set, able to support spatial statistics tests. The idea of our research is to provide a spatio-temporal analysis of ionospheric scintillation events over Equatorial regions by applying spatial statistics on ISMR Multi-GNSS measurements. In particular, by using spatial statistics, we aim to resolve specific issues regarding ionospheric scintillation data from an ISMR network established in Brazil. The research consists in spatially describing the data set, detecting and measuring potential spatial autocorrelation, determining the scale of the spatial dependency and finally producing an interpolated scintillation sky map at a given time. In terms of applicability of the methodology, our research project consists in exploiting the spatio-temporal analysis performed on ionospheric scintillation data in order to improve the performances and the reliability of Absolute GNSS Positioning algorithms under moderate ionospheric scintillation conditions. By assessing correlations existing between specific ISMR data and classic GNSS observations, the method could be extended to a more general usage which would be independent of ISMR measurements. [less ▲]

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and ... [more ▼]

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and diffraction effects. The ionospheric refraction mainly results in a modification of the propagation speed of the GNSS electromagnetic signals, inducing an error (propagation delay or phase advance depending on the observable) in GNSS measurements. In the frame of absolute positioning techniques, single-frequency algorithms usually exploit an ionospheric model to mitigate the ionospheric error while dual-frequency algorithms, such as the well-known Precise Point Positioning (PPP), take the benefit of the availability of two frequencies and the fact that the ionosphere is a dispersive medium to construct an ionosphere-free mathematical model. But these two strategies are not able to counteract the effect of the ionospheric diffraction which is due to small-scale irregularities in the free electron density. By scattering GNSS signals, these irregularities generate rapid fluctuations (scintillations) in the amplitude and phase of GNSS signals with critical consequences for GNSS applications: cycle slips, signal power fading, receiver loss of lock and poor resulting satellite geometry. The goal of our research is to develop a strategy to mitigate the effect of ionospheric scintillations on absolute GNSS positioning techniques, in particular the SPP (Standard Point Positioning) and the PPP (Precise Point Positioning). The strategy is based on the adjustment of the stochastic model. In order to construct the stochastic model (diagonal and non-diagonal elements) and study the correlation between observables, we adopted a “spatial” and an “empirical” approach. The spatial approach consists in a study of the spatial autocorrelation existing in scintillations effects on GNSS signals. The spatial autocorrelation is detected by using specific spatial analysis techniques applied on data from a network of ISMR (Ionospheric Scintillation Monitoring Receiver) stations located at equatorial and polar latitudes, where scintillations effects are most severe. The knowledge of how scintillation effects are spatially correlated is helpful for determining a coherent stochastic model. The empirical approach does not take into account the phenomenon spatiality and the locations of the measurements but only the observation data. Its objective is to determine the statistical correlation which exists between GNSS measurements during a scintillation event by using a moving filter applied on GNSS observation and scintillation data. The spatial approach exploits data and data locations while the empirical approach is based only the data itself. [less ▲]

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and ... [more ▼]

The ionosphere has always been a major limitation for GNSS positioning applications. Free electrons in the ionosphere perturb the propagation of GNSS radio signals involving both refraction and diffraction effects. The ionospheric refraction mainly results in a modification of the propagation speed of the GNSS electromagnetic signals, inducing an error (propagation delay or phase advance depending on the observable) in GNSS measurements. In the frame of absolute positioning techniques, single-frequency algorithms usually exploit an ionospheric model to mitigate the ionospheric error while dual-frequency algorithms, such as the well-known Precise Point Positioning (PPP), take the benefit of the availability of two frequencies and the fact that the ionosphere is a dispersive medium to construct an ionosphere-free mathematical model. But these two strategies are not able to counteract the effect of the ionospheric diffraction which is due to small-scale irregularities in the free electron density. By scattering GNSS signals, these irregularities generate rapid fluctuations (scintillations) in the amplitude and phase of GNSS signals with critical consequences for GNSS applications: cycle slips, signal power fading, receiver loss of lock and poor resulting satellite geometry. The goal of our research is to develop a strategy to mitigate the effect of ionospheric scintillations on absolute GNSS positioning techniques, in particular the SPP (Standard Point Positioning) and the PPP (Precise Point Positioning). The strategy is based on the adjustment of the stochastic model. In order to construct the stochastic model (diagonal and non-diagonal elements) and study the correlation between observables, we adopted a “spatial” and an “empirical” approach. The spatial approach consists in a study of the spatial autocorrelation existing in scintillations effects on GNSS signals. The spatial autocorrelation is detected by using specific spatial analysis techniques applied on data from a network of ISMR (Ionospheric Scintillation Monitoring Receiver) stations located at equatorial and polar latitudes, where scintillations effects are most severe. The knowledge of how scintillation effects are spatially correlated is helpful for determining a coherent stochastic model. The empirical approach does not take into account the phenomenon spatiality and the locations of the measurements but only the observation data. Its objective is to determine the statistical correlation which exists between GNSS measurements during a scintillation event by using a moving filter applied on GNSS observation and scintillation data. The spatial approach exploits data and data locations while the empirical approach is based only the data itself. [less ▲]

For GPS single frequency users, the ionospheric contribution to the error budget is estimated by the well-known Klobuchar algorithm. For Galileo, it will be mitigated by a global algorithm based on the ... [more ▼]

For GPS single frequency users, the ionospheric contribution to the error budget is estimated by the well-known Klobuchar algorithm. For Galileo, it will be mitigated by a global algorithm based on the NeQuick model. This algorithm relies on the adaptation of the model to slant Total Electron Content (sTEC) measurements. Although the performance specifications of these algorithms are expressed in terms of delay and TEC, the users might be more interested in their impact on positioning. Therefore, we assessed the ability of the algorithms to improve the positioning accuracy using globally distributed permanent stations for the year 2002 marked by a high level of solar activity. We present uncorrected and corrected performances, interpret these and identify potential causes for Galileo correction discrepancies. We show vertical errors dropping by 56–64 % due to the analyzed ionospheric corrections, but horizontal errors decreasing by 27 % at most. By means of a fictitious symmetric satellite distribution, we highlight the role of TEC gradients in residual errors. We describe mechanisms permitted by the Galileo correction, which combine sTEC adaptation and topside mismodeling, and limit the horizontal accuracy. Hence, we support further investigation of potential alternative ionospheric corrections. We also provide an interesting insight into the ionospheric effects possibly experienced during the next solar maximum coinciding with Galileo Initial Operation Capability. [less ▲]

The Precise Point Positioning (PPP) has become a powerful satellite positioning technique which nearly equals performances provided by advanced relative positioning techniques. Exploiting the growing availability and quality of IGS products (satellite orbit and clock products), the PPP technique can now provide a centimetre level solution in static mode and a decimetre level in kinematic mode. However, the PPP technique still presents some weaknesses. In order to reach a high precision level, it requires a significant convergence period which can typically reach 30 minutes under normal conditions. Moreover, the PPP seems to be especially sensitive to ionospheric scintillations effects which involve signal amplitude and phase variations of GNSS signals. These weaknesses still limit the use of the PPP technique in the frame of some specific and demanding applications (agricultural industry, airborne mapping, etc.). The goal of our research project is to develop new data processing strategies attempting both to make the PPP technique more reliable under ionospheric scintillations and to optimize the PPP convergence time. The project is composed of several workpackages aiming to improve the mentioned current PPP weaknesses with specific strategies. One of the workpackages is devoted to the impact of satellite geometry on PPP performances. Ionospheric scintillations are susceptible to reduce the number of tracked satellites which degrades the quality of satellite geometry. Based on an analytical development, we first attempt to figure out what types of satellite geometry can be harmful. Then, we discuss about the improvement of the satellite geometry quality involved by the combined use of GPS and Galileo and its benefits in the frame of the PPP. Another workpackage is related to the weighting scheme. Based on an iterative least-square adjustment, the PPP algorithm requires the definition of a stochastic model composed of an observation covariance matrix. Usually, this matrix is chosen as diagonal with zero covariances assuming that correlations between observations can be neglected. In particular, our project aims to study the validity of this stochastic model for the PPP in order to determine whether tuning the weighting scheme of the stochastic model can improve the PPP performances. By exploiting spatial analysis techniques, we try to characterize the spatial auto-correlation between GNSS observations, considering the signal-to-noise ratio as the main observable. From the results of these experiments, we will discuss about the spatial correlation between GNSS observations both under normal conditions and ionospheric scintillations. [less ▲]

The Precise Point Positioning (PPP) has become a powerful satellite positioning technique which nearly equals performances provided by advanced relative positioning techniques. Exploiting the growing availability and quality of IGS products (satellite orbit and clock products), the PPP technique can now provide a centimetre level solution in static mode and a decimetre level in kinematic mode. However, the PPP technique still presents some weaknesses. In order to reach a high precision level, it requires a significant convergence period which can typically reach 30 minutes under normal conditions. Moreover, the PPP seems to be especially sensitive to ionospheric scintillations effects which involve signal amplitude and phase variations of GNSS signals. These weaknesses still limit the use of the PPP technique in the frame of some specific and demanding applications (agricultural industry, airborne mapping, etc.). The goal of our research project is to develop new data processing strategies attempting both to make the PPP technique more reliable under ionospheric scintillations and to optimize the PPP convergence time. The project is composed of several workpackages aiming to improve the mentioned current PPP weaknesses with specific strategies. One of the workpackages is devoted to the impact of satellite geometry on PPP performances. Ionospheric scintillations are susceptible to reduce the number of tracked satellites which degrades the quality of satellite geometry. Based on an analytical development, we first attempt to figure out what types of satellite geometry can be harmful. Then, we discuss about the improvement of the satellite geometry quality involved by the combined use of GPS and Galileo and its benefits in the frame of the PPP. Another workpackage is related to the weighting scheme. Based on an iterative least-square adjustment, the PPP algorithm requires the definition of a stochastic model composed of an observation covariance matrix. Usually, this matrix is chosen as diagonal with zero covariances assuming that correlations between observations can be neglected. In particular, our project aims to study the validity of this stochastic model for the PPP in order to determine whether tuning the weighting scheme of the stochastic model can improve the PPP performances. By exploiting spatial analysis techniques, we try to characterize the spatial auto-correlation between GNSS observations, considering the signal-to-noise ratio as the main observable. From the results of these experiments, we will discuss about the spatial correlation between GNSS observations both under normal conditions and ionospheric scintillations. [less ▲]

in 3rd International Colloquium – Scientific and Fundamentals Aspects of the GALILEO Programme (2011, September 02)

Data preprocessing is a mandatory stage for most of GNSS applications. In the frame of space weather and precise point positioning applications, the Geomatics Unit of the University of Liège has purchased ... [more ▼]

Data preprocessing is a mandatory stage for most of GNSS applications. In the frame of space weather and precise point positioning applications, the Geomatics Unit of the University of Liège has purchased two Septentrio PolaRx3eG receivers which allow tracking GPS L1/L5 and Galileo E1/E5a signals. In order to fully exploit these new data, we developed a preprocessing method extending existing techniques. Our preprocessing method consists of three consecutive steps. The first step is devoted to the compensation of receiver clock slips affecting code pseudorange and carrier-phase measurements. The second step covers cycle slips detection and the third step assesses data quality in terms of noise essentially affecting code pseudorange measurements. This preprocessing method was initially developed for GPS L1/L5 and Galileo E1/E5a dual frequency data but finally enhanced to also preprocess triple frequency data from first operational Galileo satellites as soon as data are available. The developed method already showed promising results. [less ▲]

GPS dual frequency L1/L2 measurements have been used for many years to reconstruct the ionosphere Total Electron Content and to detect small-scale irregular structures in the ionospheric plasma. TEC is ... [more ▼]

GPS dual frequency L1/L2 measurements have been used for many years to reconstruct the ionosphere Total Electron Content and to detect small-scale irregular structures in the ionospheric plasma. TEC is usually computed by forming the geometry-free combination of L1/L2 phase measurements. Most of the existing techniques use the geometry-free combination of code measurements to solve the non-integer geometry-free ambiguity. This methodology requires the computation of satellite and receiver hardware biases. In addition, the quality of the ambiguity resolution process strongly depends on code multipath. New signals from Galileo and from modernized GPS offer new opportunities for TEC reconstruction. First attempts to compute TEC using triple frequency measurements from modernized GPS (L1/L2/L5) and from Galileo (E1, E5a, E5b) give very promising results but, at the present time, only a few triple frequency GPS/Galileo receivers are available. The University of Liege has purchased two Septentrio PolaRx3G receivers which allow tracking GPS L1/L5 and Galileo E1/E5a signals. These receivers have been installed at the Geophysical Observatory of Dourbes (50.1°N, 4.6° E) and are continuously tracking all GPS and Galileo satellites in view (including GPS SVN49/PRN01, Giove A and Giove B) since November 2009. The paper analyzes the added value of L1/L5 and E1/E5a geometry free combinations and of the new GPS and Galileo signals for TEC reconstruction. It discusses the influence of multipath and of SVN49/PRN1, Giove A and Giove B hardware biases. [less ▲]

Relative positioning with GNSS is generally used to achieve precise positions in the frame of critical applications (surveying, photo-control...). On this basis, we have developed a software which allows ... [more ▼]

Relative positioning with GNSS is generally used to achieve precise positions in the frame of critical applications (surveying, photo-control...). On this basis, we have developed a software which allows to compute a positioning error due to the ionosphere only using reference stations belonging to the Belgian Dense Network (BDN). This network consists in 66 GPS (dual-frequency) receivers over the whole Belgium. The drawback of this method is that this computation needs the design matrix which contains coefficients depending on satellite constellation geometry. Therefore, like for absolute positioning, a poor geometry (evaluated by the Dilution of Precision, or DOP) can also lead to large positioning error that cannot be separated from the one due to ionospheric effects, and in particular the small-scale structures. The main goal of this paper is to build a similar index to DOP for relative positioning in our software to be able to separate the ionospheric effects from the geometric ones. The final step is to study the feasability of a service for users of relative positioning using the BDN. The objective is to give in post-processing the positioning accuracy degradation for all BDN baselines and to associate a colour scheme to the different degradation classes created. [less ▲]

Precision of GNSS (Global Navigation Satellite System) is affected by a lot of different factors, such as satellite geometry. The quality of satellite geometry is evaluated by an indicator: DOP (Dilution ... [more ▼]

Precision of GNSS (Global Navigation Satellite System) is affected by a lot of different factors, such as satellite geometry. The quality of satellite geometry is evaluated by an indicator: DOP (Dilution Of Precision). Specific satellite geometry, such as conical satellite geometry, are able to strongly harm to the precision of positioning. These particular situations lead the normal matrix to a singular state and the DOP to high values. [less ▲]

« Google Earth » and « Windows Live Maps » are two services of web mapping recently developed by two competitive American corporations: « Google » and « Microsoft Corporation ». These services, whose working requires specific computer technologies, provide a visualization of the Earth using an assemblage of satellite images and aerial photographs. This article is written to try to establish a functional analysis and a comparison of free versions of these two services of web mapping based on their technical characteristics, qualities of their sources, levels of provided functionalities and additional possibilities of paying versions. [less ▲]